Avoid doing multiple route lookups for the same destination IP during forwarding

ip_forward() does a route lookup for testing this packet can be sent to a known destination,
it also can do another route lookup if it detects that an ICMP redirect is needed,
it forgets all of this and handovers to ip_output() to do the same lookup yet again.

This optimisation just does one route lookup during the forwarding path and handovers that to be considered by ip_output().

Differential Revision: https://reviews.freebsd.org/D2964
Approved by: ae, gnn(mentor)
MFC after: 1 week